Geophysical prospecting receptor circuits



Dec. 16, 1941. H. HOOVER, JR 2,266,041

GEOPHYSICAL PROSPECTING RECEPTOR CIRCUITS Filed April 18, 1958 2 Sheets-Sheet l INVENTOR. Z/ HERBERT HOOVER,JR.

BY KI 4 REC. n ATTORNEYS.

Dec. 16, 1941. H. HOOVER, JR I 2,265,041

GEOPHYSICAL PROSPECTING RECEPTOR CIRCUITS Filed April 18, 1938 2 Sheets-Sheet 2 I 2 3 (@L e AMP. AMP. AMP AMP. AMP. AMP.

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A B c D E fier circuits with respect to phase and amplitude.

'undesired circuits components may oppose and limit myself, in any way, to the apparatus of Patented D@. 16, 1941 2,266,041 v onornrsrou. raosrnc'rmo nnonr'ron omom'rs Herbert Hoover, Jr., Sierra Madre, CaliL, assignor to Consolidated Engin Corporation, Los

eering Angeles, Calif., a corporation of California Application April 18, 1938, Serial No. 202,752

19 Claims.

My invention relates to geophysical prospecting, and particularly to circuits having improved means for minimizing unwanted cross-feed when a plurality of pickup and amplifying circuits are used.

Among the objects of my invention are:

'To provide geophysical prospecting receptor circuits in which a plurality of receptor units may be interconnected without undesired cross-feed due to such interconnection.

To provide receptor circuits which may be cross-connected, and in which cross-feed may be controlled exactly.

To provide means for utilizing .a plurality of pickup and amplifying units simultaneously and permitting a controlled amount of cross-feed therebetween.

To provide receptor circuits into which signal components from several separate'seismometers may be introduced in desired proportions.

To provide such circuits in which the difierent signal components may be controlled as to both phase and amplitude.

I To provide means for introducing to each of a number of amplifier units signal components. controlled as to both phase and amplitude, from separate seismometers, and at the same time eliminating stray pickup and unwanted crossfeed.

To provide. means'for controlling cross-fee between several amplifiers used in coniunction with a number of seismometer units.

To provide means for controlling cross-feed between a plurality of seismometer and ampli- To provide balanced circuits of the Wheatstones bridge type for such use.

To provide balanced bridge circuits in which cancel each other.

To provide such receptor circuits in a form having a high degree of flexibility.

My invention possesses numerous other objects and features of advantage, some of which; together with the foregoing, will be set forth in the following description ofspecific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not the present application, as I may adoptvarious other apparatus embodiments, utilizing the e method, within the scope of the appended claims. I

The way in which these objects may be attained bridge circuit arrangement.

may be better understood with the aid of the drawings, wherein Fig. 1 shows a simple method of cross-connecting two seismometer and amplifier units.

Fig. 2 shows schematically. one form of my Fig. 3 illustrates a method of interconnecting a plurality of seismometer and amplifying units,

Fig. 4 shows means for controlling cross-feed in a circuit of the type of Fig. 3.

The most important geophysical prospecting method, as is well known in the art, consists in exploding dynamite at or near the surface of the ground and recording the vibration set up thereby in the earth. The waves received include some passing directly from the explosion point to the receiving apparatus, and some which have been reflected back toward the surface from various strata therebeneath. By proper interpretation of the record, it is possible to map the underlying formations with great accuracy, and derive very important information as to the location of valuable deposits. Particular success has been achieved by these means in connection with amplifier in turn actuated a galvanometer element or other recording device. The final record, therefore, was a separate reproduction of each seismometer individually.

An improvement was outlined in Taylor Patent No. 1,799,398 where two or more seismometers were used on ea h amplifier and galvanometer". Each galvano eter, according to this patent, recorded the algebraic sum of the voltages generated by all the'seismomoters attached to that particular amplifier.

Still another improvement was outlined in North Patent No. 2,059,018. One common method of using this patent in actualpractice is to connect part or all of seismometers in series.

The inputs to the individual amplifiers are taken off across groups of the seismometers, in such a way that one or more seismometers may be common to two or more amplifiers. One example of the almost infinite number of possible circuits is shown in Figure 3. This is generally termed "over-lapping" or mixing.

The systems such as shown in Figure 3 are subject to considerable unwanted cross-feed and stray pickup, which may be reduced by inserting isolation transformers in the circuits, as disclosed in my co-pending application for United States Letters Patent, Serial No. 164,100, filed September 16, 1937, and entitled Geophysical prospecting receptor circuits.

The elementary circuits involved may be seen in simplified form in Figure 1. In this connection it should be understood that any desired number of additional seismometer and amplifying units may be added, each connected to its predecessor as indicated by the dotted lines.

In Figure 1, I, 2, 3, and 4 are seismometer units, A, B, C, and D being the amplifying and recording units pertinent thereto. The amplifying and recording units A, B, C and D have impedance-matching transformers 2|, 22, 23 and 24 connected to their respective inputs. The amplifying and recording unit A is inductively linked to the corresponding seismometer I through an isolation transformer II and an impedance matching transformer 2|. The isolation transformer II has a primary I1 and a secondary I8. The impedance matching transformer 2| has a primary 28, and a secondary 29.

Seismometer I is connected to isolation transformer primary I1, isolation transformer secondary I8 is connected to impedance matching transformer primary 28, and the impedance matching transformer secondary 29 is connected to the amplifying and recording unit A.

Similarly, the seismometers 2, 3 and 4 are connected to the corresponding amplifying and recording units B, C and D through corresponding isolation transformers I2, I3 and I4. The secondary I8 is connected to the primary of the impedance matching transformer 22 in series with the secondary of isolation transformer I I through leads and 3|. The secondary I8" of transformer I3 is connected with the primary of impedance matching transformer 23 in series with the secondary I8 of isolation transformer I2 through leads 30' and 3I'. The secondary I8 of transformer I4 is connected to the primary of impedance matching transformer 24 in series with the secondary I8" of isolation transformer I3 through leads 30" and 3|". If another pickup and recording group is desired, connection would be made as indicated by the dotted lines 32 and 33, corresponding to leads 30 and 3|, etc., shown, the method being capable of indefinite expansion. It is also sometimes desirable to ground the midpoint of the isolation transformer primaries, and to use an electrostatic shield in both the isolation transformers and matching transformers,- the shield being grounded. These additions are omitted from Figure 1 for clarity, and are shown and discussed further in connection with Figure 4 later.

It is apparent that in Figure 3, it is intended that amplifier and recorder A shall receive only the output of seismometer I; that B shall receive substantially equal outputs from I and 2; that C shall receive substantially equal outputs from 22 and 3; that D shall receive substantially equal outputs from 3 and 4; and so on for other channels similarly connected to the foregoing.

Several advantageous effects are claimed for this and similar arrangements, namely that each seismometer is in efiect used more than once, and that therefore more eificient use is obtained from a given number of them; that some out-of-phase cancellation (as outlined in the Taylor patent) of unwanted surface waves is obtained; and that incoming reflections tend to line up better due to the fact that each recorded trace represents an overlapped average of one or more adjacent traces. Briefly stated, the effect is to introduce a definite and controlled amount of cross-feed.

A critical analysis of the circuits, however, will show that, for instance, channel D will not only receive energy from seismometers, 3 and 4 (the desired condition), but in practice will receive appreciable energy from seismometers considerably removed from those to which the effect should be confined. This shows up as apparent unwanted cross-feed between channels. The effect is particularly serious because the unwanted cross-feed is often out-of-phase with the desired cross-feed.

To demonstrate this point, assume that an ima pulse is received on seismometer 3 in the circuit of Figure 1. The impulse will appear in the secondary I8" of the corresponding isolation transformer and from there find its way to the primaries of the matching transformers associated with recording units C and D; thence being amplified and recorded by channels 0 and D in substantially equal strength. This is the desired condition, and represents what is generally called 50% overlap.

A further analysis of the circuit, however, shows that the impulse will also find its way into some, or all, of the other channels in varying amounts. Though usually this unwanted cross-feed signal to other traces is substantially less than to the desired traces, nevertheless it is often of such magnitude and relative phase as to seriously interfere with the desired results.

Take the above exampl: of an impulse from seismometer 3. The impulse appears in the secondary of the associated isolation transformer. In order to get to the primary of the matching transformer 23 and thence to the amplifier C, it is necessary that it also pass through the secondary I8 of the isolation transformer I2, the primary of which is connected to seismometer 3. This means that the impulse is common to the circuit including the secondaries of the isolation transformers associated with seismometers I and 2 and the primary of the matching transformer associated with unit B and will therefore appear in amplifier B and be recorded by that trace.

The amplitude appearing in B will be somewhat less than in C and D, but nevertheless will be appreciable. It is also important to note that while the phase of the signal in C and D will be the same, and the recordings will appear to be similar, that nevertheless the unwanted crossfeed in channel 2 will be 180 out-of-phase, and will appear upside down on the recording, and may seriously interfere with any impulses received by seismometer 3 and transmitted to chan nel B at approximately the same time.

By similar analysis it can be shown that appreciable (although diminished) unwanted crossfeed will appear in channel A, and that it also will be 180 out-of-phase with the impulses recorded in C and D. By carrying the analysis even further, it can be shown that unwanted cross-feed, in constantly diminishing amounts, will be experienced in channels at a considerable distance on each side, and that the phase is reversed by essentially 180 each time that it passes from one circuit to the next.

One method of reducing the above type of crossfeed has been to make the input impedances of.

the amplifiers, represented in Figure l by the primaries 28-28, etc., of transformers 2!, 22, etc., as high as possible, either for the direct purpose of eliminating cross-feed, or incidental to other purposes such as control of volume. In

the examples cited above where the impulse was introduced through seismometer 3, this would reduce the amount of voltage drop across the secondaries of the adjacent isolation transformers, and thus reduce the amount of unwanted cross-feed starting into the adjacent channels. It 'is often not. convenient to carry this practice beyond a certain point, however, because of the dimculties in getting an efficient voltage transfer into the amplifiers, and for-other reasons, such as the stray-coupling of high impedance windings, connecting leads, etc.

Another specific method previously used is to employ transformers having high impedance secondaries suitable for working directly into the grid of an amplifier tube. The or overlapping may then be affected directly in series with the grid circuit of the amplifier, and because of the high load impedance reflected by the grid, very little cross-feed will result. Still other methods use separate tubes in the amplifier for each circuit to be mixed, the plates of these tubes being connected substantially in -parallel. Most of these systems, however, are relatively inflexible, and flexible switch arrangements are difficult to obtain from them.

The method proposed-here is to makeeach of the circuits a Wheatstone bridge type network. In Figure 1, neglecting now the dotted lines 32 and 33, there is overlap introduced between the two channels according to conventional practice. Assume it is desired to have channel A respond only to seismometer i, and channel B respond to both seismometers i and 2. As shown in the previous analysis, some unwanted cross-feed fromseismometer 2 will be recorded on channel A, and it will be 180 out-of-phase with that recorded on channel B. The desirable condition is to eliminate unwanted cross-feed altogether, or under some conditions possibly to deliberately introduce some cross-feed in phase. For this purpose, l have developed a modification of a Wheatstone bridge circuit. There are several configurations able to give the desired results, one of which is shown schematically in Figure 2, which derives fromFigure 1.

In Figure 2, the secondary it? of isolation transformer i ll has a center tap 35, and end terminals 36 and 3'11 to which is joined the primary 28 of matching transformer 2i.

Between terminals 35 and 31 there is also a current path through the primary 28' of matching transformer 22 and a variable resistor 39 connected in series therewith between the said terminals. Across center tap 35 on secondary it of transformer M and a point 50 common to resistor 39 and primary 28, I connect the secondary it of isolation transformer !2. There are also connected in the circuit two fixed resistors, Lid, across the primary 2% of matching transformer 2i, and M across the primary 28' of matching transformer 22.

It will be seen that the circuit of Figure 2 is of the Wheatstones bridge type, with the segment of transformer secondary l8 lying between terminals 35 and 36, together with matching transformer primary 28' and its shunt resistance 36, forming one branch, and the transformer segment lying between terminals .35 and 3'! forming, together with variable resistor 39, the opposed branch.

The conjugate arms are formed between terminals 35 and 30 by secondary l8, and between terminals 36 and 31 by primary 28 and its shunt resistor id.

It will be obvious to those skilled in the art I siderable reactance, and it is to render this re-,

actance component less effective that resistors 65 and M are used. it is to be understood, of course, that resistor 39 may be any other type of impedance such as an inductance or other balancing impedance. Thus a more nearly perfect impedance balance may be obtained.

By adjustment of resistor 39, it is also possible to unbalance the bridge so that a desired and controlled amount of cross-feed is obtained from seismometer 2 to recorder A,'and by properly proportioning the fixed shunts 66 and 46' it is possible to exercise phase control as well, so that signals from a number of seismometer units may be introduced in phase or phase opposition as desired, and the component from each which enters a particular recorder maybe exactly predetermined.

In Fig. 2 a current due to seismometer 2 produced in secondary 58 of transformer 52 flows upward to center tap 35 of secondary i8 where it is divided into two substantially equal parts, one flowing to the left and one to the right. Each of these branch currents produces equal and opposite magnetic fields in the core of transformer II and due to the mutual inductance of the two halves of secondary M the resultant potential across each half of secondary i8 is substantially zero. Transformer H is substantially non-induce tive so far as currents from transformer 52 are concerned. This is especially true if the transformer is of the closed core type.

So far as currents from seismometer 2 are concerned the effective impedance between the midpoint 35 and either terminal of transformer l l is therefore very small, being due almost entirely to the copper loss'in the winding. The potential drop, therefore, between the midpoint 35 and either outside terminal of the secondary it due to waves arriving at seismometer 2 is very small compared to the potential drop across impedance 39 or the substantially equal potential drop across the parallel network comprising resistor 66' and primary 28'.-

If, therefore, recorder A is disconnected from terminal 31, and this same lead from recorder Ais connected to any other point of the secondary it, such as the midpoint 35, very little current from secondary It will actuate recorder A. Such a shift of connection is seen in Fig. 4. This phenomenon may be regarded several ways: first of all, the potential drop from secondary l8 across any portion of the secondary i8 is very small;

' secondly the 'efi ective impedance of winding it as viewed from winding l8 practically short circuits the primary of the transformer 28; looked at still a third way, the potential that would ordinarily exist between points 35 and either 36 or 3? due to currents from secondary l8 are counteracted by appropriate induced voltages or bucking voltages.

The derived bridge type circuit hereinabove described has been used in Fig. 4 to provide controlled coupling and crossfeed between various receptor circuits.

In Figure 4, I have shown the Wheatstone type bridge circuit derived from Figure 2 applied to a plurality of channels, six being shown; others can be added in similar fashion. This circuit per-' forms the same function as the circuits of Figures 1 and 3, plus the control features of Figure 2. To ach of the seismometer units I--6, I connect t e primary III1 of an isolation transformer I II6, and preferably ground the midpoints, 4I-4I Between the primary windings I'I--II and secondaries I8I8 electrostatic shields'S I5 I are interposed, and also grounded.

Variable balancing resistors 39-39 are pro-' those connections made to secondaries I8I8 In case no further channels are added, the right half of transformer secondary I8 to which the lead 65 is connected may be, omitted. Or if desired for symmetry an impedance equal to resistance 39 plus another recorder may be connected in series across leads 65 and 86.

To understand how limited crossfeed between channels is obtained when the bridge type circuits are in balance, consider receptor number 2. When earth waves are received, voltage is generated across the secondary I8 of transformer III.

In the preferred application of my invention resistor 39' has approximately the same impedance value as the parallel network 28"4 4 so that the common point between them connected to the center tap of secondary I8" is at substantially the same potential as the mid-point of the secondary I8. Since the secondary I8- of transformer I3 is connected to the two aforementioned equi-potential points, no appreciable part of the voltages appearing across the secondary I8'of transformer I2 can affect other circuits connected Therefore, the waves received to transformer I 3. at seismometer number 2 do'not affect recorders D, E, or F in any appreciable amount.

It will now be shown that the waves arriving at seismometer number 2 affects recorder B but not recorder A. Current is generated in the left half section of the secondary I8 of the transformer I2. Here the current is divided into two substantially equal parts, one going to the left and returning to the midpoint of secondary I8 through the resistor 44' and primary 28' and the 1 other going to the right and returning to the midpoint of secondary I8 through the resistor 39. Waves received by receptor number 2 therefore actuate record B. Since secondary I8 virtually acts as a short circuit across primary 28 so far as currents from transformer I2 are concerned-as hereinabove explained in connection with Fig. 2,

actuates recorder A.

It should be observed that a signal from seismometer 2 would reach recorder A were it not for thebuclzing voltage introduced into the left Matching transformer secondaries 2929 half of winding I8 by currents flowing in the right half of winding I8. It is obvious that such bucking voltage may be introduced by other methods, but the use of the auto-transformer connections of secondary I8 are considered preferable.

It is interesting to note that regardless of the currents flowing in any of the circuits the voltages across the two halves of each secondary I8I 8 are always substantially equal,-due to the auto-transformer efiects of these windings.

By a similar analysis of the other circuits of Fig. 4, it is readily seen that in the balanced condition each seismometer actuates no more than two recorders. Recorder A, of course, is actuated by only the single receptor I.

The efiectiveness of the network of Fig, 4 in reducing the amplitude of unwanted crossfeed is in part due to the modified Wheatstone bridge principle and partly -'due to the application of a system of inductive bucking voltages. In this network I have used a sequence of modified Wheatstone bridges each within another and taken advantage of these bucking voltages.

In a sense the Wheatstone bridge type circuits,

too, utilize a set of bucking voltages for preventcorder C. Similar reasoning applies, of course, to

the other receptor circuits and reproducers.

Looked at broadly, therefore, my invention utilizes circuits which produce desired crossfeed only a negligible current from seismometer. Z

but simultaneously prevent undesired crossfeed by the introduction of bucking voltages.

Recorders A and B are associated with seismometer I, B and C with 2, C and D with 3, etc. When the bridge type circuits are balanced, signals from each seismometer reach only the recorders associated therewith. The cross-feed between channels associated With seismometers I and 2 is controlled by resistor 39, that between channels associated with seisr'nometers I and 3 by resistors 39 and 39', and so on.

The method and circuit here shown is not the only means of obtaining the desired result, as the mixing and controlling may be done at other points in the system, for example, in the output circuit between the amplifiers and recorders, using a similar arrangement, or neutralizing by means of bucking out the unwanted signals maybe had.

Applying the reciprocatory theorem and the principle of superposition to the circuits of Figs.

2 9 29 and receptors I 6 are interchanged with recorders F A and the associated amplifiers respectively, then the relation between the group of traces appearing on the recorders A F and the waves reaching receptors I 6,

will be the same as in the circuit of Fig. 4. For

example, if the bridge type circuits are balanced, recorder A-will reproduce signals from receptor I alone, recorder B will reproduce signals from both receptors I and 2, etc., as in Fig. 4.

It will be apparent to those versed in the art that the balancing and cross-feed controlling system here presented may be combined with the switching arrangement disclosed in my pending application, cited supra, to form an extremely flexible and perfectly controlled prospecting device.

By the circuits and methods outlined above, I have provided means for taking advantage of the connection together of a number of seismometer units, applying their output cumulatively or in opposition as desired to recording units, with means for eliminating cross-feed entirely when desired, or permitting a controlled amount wherever useful, and also controlling the phase relations between different channels.

The term receptor as herein used shall be deemed to include devices known as seismometers, detectors, or pickup instruments, and also to include any and all devices or means to convert earth vibrations into energy suitable for use in 2 making a record of the changes in the earths condition due to an explosion or other vibration producing impact. The term receptor also includes any device whatever used for receiving electrical waves or other impulses used in geophysical exploration.

I claim: 1. In a geophysical prospecting system, a plurality of receptors, a corresponding plurality of receptor lines, a corresponding plurality of amplifier and recorder units operable in accordance with electric signals arriving over said lines, an impedance matching device between each of said lines and the corresponding amplifying and recording unit, an isolation transformer between each of said lines and the corresponding impedance matching device, and means including impedances interconnecting isolation transformer secondaries and linking portions of connected transformers in series relationship so that por- 4o tions of adjacent connected transformers are. parts of a common current carrying circuit, said circuit also including a connecting "impedance, all said irnpedances and said secondaries being so connected and arranged that a controlled 5 amount of crossfeed is introduced between at least one of said secondaries and only a fraction of the remaining interconnected secondaries.

2. In a geophysical prospecting system having a plurality of receptors and a corresponding number of receptor lines, a plurality of amplifier and recorder units operable in response to.

signals received on said lines, an impedance matching device between each of said lines and the corresponding amplifier and recorder unit, an isolation transformer between each of said lines and the corresponding impedance matching device, said impedance matching device comprising a matching transformer hgving a primary variable resistor.

3. In a geophysical prospecting system, a plurality of receptors, a corresponding plurality of receptor lines, a eorresponding'plurality of am- 7 plifier and recorder units operable in accordance with electric signals arriving over said lines, an

impedance matching device between each of said lines and the corresponding amplifying and recording unit, an isolation transformer between isolation transformer secondaries, im- 5 each of said lines and the corresponding impedance matching device, said means comprising a bridge circuit having half of twoopposed arms formed by an isolation transformer secondary, having the remaining half of one opposed arm formed by an impedance matching transformer primary, and having the remaining half of a second opposed arm formed by a variable resistor,

' having one conjugate arm formed by the sec? ondary of a second isolation transformer and having a second conjugate arm formed by the primary of a second matching transformer.

4. A geophysical prospecting circuit substantially as described in claim 3, whereinthe midpoint of each isolation transformer primary is grounded.

5. A geophysical prospecting circuit substantially as described in claim 3, wherein the midpoint of each isolation transformer primary is grounded and wherein a grounded shield is provided between the primary and secondary windings of each of said isolation transformers.

- 6. A geophysical prospecting circuit substantially as described in claim 3, wherein the midpoint of each isolation transformer primary is grounded and wherein a grounded shield is provided between the primary and secondary windings of each of said isolation transformers, and wherein grounded electrostatic shields are positioned between the primary and secondary windings of each of said impedance matching transformers.

'7. In a geophysical prospecting circuit substantially as described in claim 3, fixed resistors shunted across the primary of each of said impedance matching transformers.

8. Geophysical prospecting circuits substantially as described in claim 3 having a plurality of associated units, connected to form a plurality of bridge circuits between adjacent units.

9. A geophysical prospecting system, comprising a plurality of receptor circuits, each of said circuits including at least one receptor, a corresponding plurality of recorders, mixing means providing channels intermediate said receptor circuitsand said recorders; for introducing signals from the receptor circuits to the respective corresponding recorders, and'auxiliary circuits connected to introduce desired crossfeed, between certain receptor circuits and certain non-corresponding recorders, said auxiliary circuits including inductive means for preventing undesired crossfeed between said certain receptor circuits and other non-corresponding recorders.

10. In a geophysical prospecting system, aplurality of signal transmission lines, each of said lines being connected to at least one receptor, a plurality of reproducers operable in accordance with signals received by said receptors, and mixing circuits intermediate said lines and said reproducers; said mixing circuits comprising means for introducing desired crossfeed from certain lines to certain reproducers, and inductive means interconnecting mixing circuits to introduce bucking voltages from some of said lines to some of said mixing circuits to prevent undesired crossfeed between said certain lines and other mixing circuits.

11. In a geophysical prospecting system, a plurality of signal transmission lines, each of said lines being connected to at least one receptor, a plurality of reproducing. circuits operable in accordance with signals received by said receptors, and mixing circuits intermediate said lines and said reproducing circuits, said mixing circuits comprising current transfer means directly connected to produce desired crossfeed between certain lines and certain reproducing circuits and inductive means for simultaneously introducing bucking voltages from some lines to some reproducing circuits to prevent undesired crossfeed.

12. In a geophysical prospecting system a plurality of signal transmission lines, each of said lines being connected to at least one receptor, a plurality of reproducersoperable in accordance with signals received by said receptors, and mixing means connected intermediate said lines and said reproducers, said mixing means compris ing input elements connected to said lines, balancing impedances, and overlapping bridge type circuits interconnecting said balancing impedances, said reproducers, and said input elements.

13. A geophysical prospecting system comprising receptors, recorders, and amplifiers, if necessary; a sequence of first transformer windings each having first and second end taps, and center taps; a conductive connection from the cen ter tap of each of said transformer windings, except the last, to the first end tap of the next transformer winding following in sequence; a balancing impedanceconnecting the center tap of each transformer winding, except the first, with the second end-tap of the next of said transformer windings prior in sequence; a corresponding sequence of second transformer' windings; conductive connections from end terminals'of each *of said second transformer windings to first end taps of two alternate non-adjacent first' transformer windings; means couplingsaid'receptors'to. one of said sequence of trans-' former windings, means coupling said recorders to the other of said sequence of transformer windings.

14. A geophysical prospecting circuit as in prising receptors and reproducers; a sequence of first transformer windings each of said transformer windings having a center tap and a first and second end taps; a conductive connection from thecenter tap of each of said transformer windings, except the last, to the first end tap of the next transformer winding following in sequence; an impedance connecting the center tap of each transformer windings, except the first, with the second end tap of the next of said transformer winding prior in sequence; a corresponding sequence of second transformer windings, having first and second terminals; a conductive connection from each of said first end flaps to the first terminal of the next adjacent ing a bridge type circuit, a first pair of coupling devices comprising self-inductances, a second pair of coupling devices, means connecting one of said second coupling devices and a balancing impedance in series with one of said first coupling-devices,means connecting the other of said first coupling devices between, the mid-point of said first coupling device and the common connecting point between said second coupling device and said balancing impedance, means conmeeting the other of said second coupling devices across a portion of said one of said first coupling devices, at least one receptor connected to each of one pair of coupling devices and a reproducing unit connected to each of the other of said pairs of coupling devices.

18. In' a geophysical prospecting system, the combination which comprises at leastthree receptor circuits, 9. corresponding plurality of recorders, intermediate current-carrying circuits coupling each said receptor circuit with the said corresponding recorder, means including 'electrical impedance elements coupling each of said intermediate circuits-to at least one other of said intermediate circuits, said impedances and intermediate circuits being so connected and arrangedas to introduce a controlled amount of ceptor circuits, a corresponding plurality of recorders, common current-carrying circuits coupling each of said recorders with each of the corresponding receptor circuits, signal mixing means including impedances coupling adjacent current-carrying circuits, and means connected in said adjacent current carrying circuits for preventing cross feed from passing from one of said current carrying circuits to another of said current carrying circuits that is non-adjacent thereto.

HERBERT HOOVER, JR.

CERTIFICATE OF CORRECTION. Patent No, 2,266,ou1. December 16, 19in.

HERBERT HOOVER, J

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 62, for "Figure 5" read -Figure l--; page second column, line 56, for iprocato read --reciprocity-; page 5, second column, line l claim 9, strike out the semi-colon after "recorders"; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 19th day of May, A. D. 191m.

Henry Van Arsdale, (Seal) Acting Conmqissioner of Patents.

CERTIFICATE OF CORRECTION. Patent No. 2,266,0hl. December 16, 19M.

HERBERT HOOVER,

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 62, for "Figure 5" read --Figure 1--; page 14., second column, li e 56, for "reciprocat read --reciprocity--; page 5, second column, line 1 6, claim 9, strike out the semi-colon after "recorders"; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

7 Signed and sealed this 19th day of May, A. D. 19L 2.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents. 

